The subject invention relates to refrigeration systems. More particularly, the subject invention relates to cascade air conditioning systems with a two-phase refrigerant loop.
Refrigerant systems are known in the HVAC&R (heating, ventilation, air conditioning and refrigeration) art, and operate to compress and circulate a heat transfer fluid throughout a closed-loop heat transfer fluid circuit connecting a plurality of components, to transfer heat away from a secondary fluid to be delivered to a climate-controlled space. In a basic refrigerant system, heat transfer fluid is compressed in a compressor from a lower to a higher pressure and delivered to a downstream heat rejection heat exchanger, commonly referred to as a condenser for applications where the fluid is sub-critical and the heat rejection heat exchanger also serves to condense heat transfer fluid from a gas state to a liquid state. From the heat rejection heat exchanger, where heat is typically transferred from the heat transfer fluid to ambient environment, high-pressure heat transfer fluid flows to an expansion device where it is expanded to a lower pressure and temperature and then is routed to an evaporator, where heat transfer fluid cools a secondary fluid to be delivered to the conditioned environment. From the evaporator, heat transfer fluid is returned to the compressor. One common example of refrigerant systems is an air conditioning system, which operates to condition (cool and often dehumidify) air to be delivered into a climate-controlled zone or space. Other examples may include heat pumps and refrigeration systems for various applications requiring refrigerated environments.
Historically, conventional HFC and HCFC heat transfer fluids such as R22, R123, R407C, R134a, R410A and R404A, have been utilized in heating, air conditioning, and refrigeration applications. Recently, however, concerns about global warming and, in some cases, ozone depletion, have created a need for alternative heat transfer fluids. In some cases, the use of natural heat transfer fluids such as R744 (CO2), R718 (water), or R717 (ammonia) has been proposed. The various known and proposed heat transfer fluids each have their own advantages and disadvantages. For example, CO2 as a heat transfer fluid offers zero ozone depletion potential and low global warming potential compared to many hydrocarbon-based heat transfer fluids. However, many proposed systems having CO2 as a heat transfer fluid require the CO2 to be maintained in a supercritical fluid state, which can add to equipment and operating complexity and cost. For example, in many systems, the CO2 is subcooled, or cooled below its saturation temperature, upstream of a pump inlet between about 1.5 and 3 degrees Fahrenheit to force complete phase change of the CO2 to liquid. To reduce power consumption of the system, subcooling at the pump inlet can be eliminated, but vapor entrained in the CO2 fluid stream causes cavitation in the pump and therefore instability of the pump operation.